Natural resources such as gas, oil, minerals, and water residing in a subterranean formation can be recovered by drilling wells into the formation. For example, methane can be recovered from subterranean formations comprising of coal beds. The fluids in the subterranean formation are driven into the wells by, for example, pressure gradients that exist between the formation and the wells, the force of gravity, displacement of the fluids using pumps or the force of another fluid injected into the wells. The production of such fluids is commonly increased by hydraulically fracturing the subterranean formations. That is, a viscous fracturing fluid is pumped into a well to a subterranean formation at a rate and a pressure sufficient to form fractures that extend into the formation, providing additional pathways through which the fluids can flow to the wells.
The fracturing fluid is usually a water-based fluid containing a gelling agent to increase the viscosity of the fluid. The gelling agent reduces the leakage of liquid from the fractures into the subterranean formation. The gelling agent is commonly a polymeric material that absorbs water and forms a gel as it undergoes hydration. In addition, particulate matter known as a proppant, e.g., graded sand, bauxite, or resin coated sand, is typically dispersed throughout the fracturing fluid. The proppant is suspended in the fracturing fluid and becomes deposited into the fractures created by the pressure exerted on the fracturing fluid. The presence of the proppant in the fractures holds the fractures open after the pressure has been released. Otherwise, the fractures would close, rendering the fracturing operation useless. Ideally, the proppant has sufficient compressive strength to resist crushing.
Fracturing fluid has been developed for its proppant carrying capacity, fluid loss control, and high viscosity. As a result, a variety of fracturing fluids have been developed and used, including foamed fracturing fluids. Foamed fracturing fluids comprise a relatively large volume of gas dispersed in a relatively small volume of liquid and a surfactant for facilitating the foaming and stabilization of the foam produced when the gas is mixed with the liquid. The most commonly used gases for foamed fracturing fluids are nitrogen and carbon dioxide because they are non-combustible, readily available, and relatively cheap.
The procedure used to fracture a subterranean formation with a foamed fracturing fluid includes pumping the foamed fracturing fluid into a well at a pressure sufficient to fracture the formation. The pressure on the well is then relieved at the wellhead to reduce or defoam the fracturing fluid, releasing proppant suspended therein into the resulting fractures. This reduction of the pressure on the foam results in the fluid being carried back into the well by the rush of expanding gas. The proppant must be properly placed in the formation to adequately increase production flow from the formation. However, controlling the defoaming of the fracturing fluid to properly release the proppant can be difficult. A need therefore exists to develop an improved technique for defoaming the fracturing fluid.